Method and apparatus for transmitting channel state information in wireless communication system

ABSTRACT

A method for receiving channel state information (CSI) by a base station, includes transmitting configuration information indicating whether to use a 4 antenna ports codebook, wherein a first codebook index for a first precoding matrix indicator (PMI) and a second codebook index for a second PMI for a specific CSI report mode with 4 antenna ports are determined when the UE is configured to use the 4 antenna ports codebook; and receiving the CSI indicating the first codebook index and the second codebook index, wherein a value of the first codebook index is determined as follows: when a rank indicator (RI) is 1, the value of the first codebook index is determined based on a value of the first PMI multiplied by 4, and when the RI is 2, the value of the first codebook index is determined based on the value of the first PMI multiplied by 4.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.14/767,825 filed on Aug. 13, 2015, which is the National Phase ofPCT/KR2014/005000 filed on Jun. 5, 2014, which claims priority under 35U.S.C. 119(e) to U.S. Provisional Application No. 61/831,151 filed onJun. 5, 2013, all of these applications are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting channelstate information using subsampling in a wireless communication system.

Discussion of the Related Art

A 3rd generation partnership project long term evolution (3GPP LTE)communication system will be described below as an exemplary mobilecommunication system to which the present invention is applicable.

FIG. 1 is a diagram schematically showing a network structure of anevolved universal mobile telecommunications system (E-UMTS) as anexemplary radio communication system. The E-UMTS system has evolved fromthe conventional UMTS system and basic standardization thereof iscurrently underway in the 3GPP. The E-UMTS may be generally referred toas a long term evolution (LTE) system. For details of the technicalspecifications of the UMTS and E-UMTS, refer to Release 7 and Release 8of “3rd generation partnership project; technical specification groupradio access network”.

Referring to FIG. 1, the E-UMTS includes a user equipment (UE), eNBs (oreNode Bs or base stations), and an access gateway (AG) which is locatedat an end of a network (E-UTRAN) and connected to an external network.The eNBs may simultaneously transmit multiple data streams for abroadcast service, a multicast service, and/or a unicast service.

One or more cells may exist per eNB. A cell is set to use one ofbandwidths of 1.25, 2.5, 5, 10, 15, and 20 MHz to provide a downlink oruplink transport service to several UEs. Different cells may be set toprovide different bandwidths. The eNB controls data transmission andreception for a plurality of UEs. The eNB transmits downlink schedulinginformation with respect to downlink data to notify a corresponding UEof a time/frequency domain in which data is to be transmitted, coding,data size, and hybrid automatic repeat and request (HARQ)-relatedinformation. In addition, the eNB transmits uplink schedulinginformation with respect to UL data to a corresponding UE to inform theUE of an available time/frequency domain, coding, data size, andHARQ-related information. An interface for transmitting user traffic orcontrol traffic may be used between eNBs. A core network (CN) mayinclude the AG, a network node for user registration of the UE, and thelike. The AG manages mobility of a UE on a tracking area (TA) basis,wherein one TA includes a plurality of cells.

Although radio communication technology has been developed up to LTEbased on wideband code division multiple access (WCDMA), the demands andexpectations of users and providers continue to increase. In addition,since other radio access technologies continue to be developed, newtechnology is required to secure competitiveness in the future. Forexample, decrease of cost per bit, increase of service availability,flexible use of a frequency band, simple structure, open interface, andsuitable power consumption by a UE are required.

Multiple-input multiple-output (MIMO) technology refers to a method forenhancing transmission and receiving data efficiency by employingmultiple transmit antennas and multiple receive antennas instead of onetransmit antenna and one receive antenna. That is, the MIMO technologyenhances capacity or improves performance using multiple antennas in atransmitting end or a receiving end of a wireless communication system.The MIMO technology may also be referred to as multiple antennatechnology.

In order to support multiple antenna transmission, a precoding matrixfor appropriately distributing transmitted information according to achannel situation and so on can be applied to each antenna.

SUMMARY OF THE INVENTION

An object of the present invention devised to solve the problem lies ina method and apparatus for transmitting channel state information in awireless communication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

The object of the present invention can be achieved by providing amethod for transmitting channel state information (CSI) by a userequipment in a wireless communication system, the method includingsubsampling a first codebook associated with a first precoding matrixindicator (PMI) and a second codebook associated with a second PMIaccording to a report submode for a 4 antenna port, and reportingchannel state information based on the subsampled first codebook andsecond codebook, wherein, when a rank indictor (RI) is 1 or 2, a firstcodebook index for the first PMI is determined as one of 0, 4, 8, and12, when the RI is 1, a second codebook index for the second PMI isdetermined as one of 0, 2, 8, and 10, and when the RI is 2, the secondcodebook index for the second PMI is determined as one of 0, 1, 4, and5.

In another aspect of the present invention, provided herein is a userequipment for transmitting channel state information (CSI) in a wirelesscommunication system, the user equipment including a radio frequency(RF) unit, and a processor, wherein, the processor is configured tosubsample a first codebook associated with a first precoding matrixindicator (PMI) and a second codebook associated with a second PMIaccording to a report submode for a 4 antenna port and to report channelstate information based on the subsampled first codebook and secondcodebook, when a rank indictor (RI) is 1 or 2, a first codebook indexfor the first PMI is determined as one of 0, 4, 8, and 12, when the RIis 1, a second codebook index for the second PMI is determined as one of0, 2, 8, and 10, and when the RI is 2, the second codebook index for thesecond PMI is determined as one of 0, 1, 4, and 5.

The following features may be commonly applied to the above embodimentsof the present invention.

When the RI is 3, the second codebook index for the second PMI may haveone of integers from 0 to 15.

When the RI is 4, the second codebook index for the second PMI may haveone of integers from 0 to 15.

When the RI is 1, the second codebook index may be determined using

2I _(PMI2) +└I _(PMI2)/2┘

The IPMI2 may have an integer from 0 to 3 and indicate a value of thesecond PMI.

When RI is 2, the second codebook index may be determined using

I _(PMI2)+2·└I _(PMI2)/2┘.

The IPMI2 may have an integer from 0 to 3 and indicate a value of thesecond PMI.

The first PMI may correspond to wideband and long-term PMI, the secondPMI may correspond to subband and short-term PMI, and a final PMI may bedetermined according to the first PMI and the second PMI.

The report submode may be a second submode of physical uplink controlchannel (PUCCH) mode 1-1 for reporting PMI and a wideband channelquality indicator (CQI).

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

According to embodiments of the present invention, a method andapparatus for effectively transmitting channel state information usingsubsampling in a wireless communication system is provided.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a diagram schematically showing a network structure of anevolved universal mobile telecommunications system (E-UMTS) as anexemplary radio communication system;

FIG. 2 is a diagram illustrating a control plane and a user plane of aradio interface protocol between a UE and an evolved universalterrestrial radio access network (E-UTRAN) based on a 3rd generationpartnership project (3GPP) radio access network standard;

FIG. 3 is a diagram showing physical channels used in a 3GPP system anda general signal transmission method using the same;

FIG. 4 is a diagram illustrating an example of the structure of a radioframe used in a long term evolution (LTE) system;

FIG. 5 is a diagram illustrating a control channel included in a controlregion of a subframe in a downlink radio frame;

FIG. 6 is a diagram illustrating an uplink subframe structure used in anLTE system;

FIG. 7 illustrates the configuration of a typical multiple inputmultiple output (MIMO) communication system;

FIGS. 8 to 11 illustrate periodic reporting of channel state information(CSI);

FIG. 12 is a diagram illustrating periodic reporting of channel stateinformation discussed in an LTE-A system;

FIG. 13 is a diagram illustrating CSI feedback in submode 1 of mode 1-1of FIG. 8;

FIG. 14 is a diagram illustrating CSI feedback in submode 2 of mode 1-1of FIG. 8;

FIG. 15 is a diagram illustrating CSI feedback in mode 2-1 of FIG. 8;

FIG. 16 is a flowchart of a channel state information reporting methodaccording to an embodiment of the present invention; and

FIG. 17 is a diagram illustrating a BS and a UE to which an embodimentof the present invention is applicable.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the structures, operations, and other features of thepresent invention will be understood readily from the embodiments of thepresent invention, examples of which are described with reference to theaccompanying drawings. The embodiments which will be described below areexamples in which the technical features of the present invention areapplied to a 3GPP system.

Although the embodiments of the present invention will be describedbased on an LTE system and an LTE-Advanced (LTE-A) system, the LTEsystem and the LTE-A system are only exemplary and the embodiments ofthe present invention can be applied to all communication systemscorresponding to the aforementioned definition. In addition, althoughthe embodiments of the present invention will herein be described basedon Frequency Division Duplex (FDD) mode, the FDD mode is only exemplaryand the embodiments of the present invention can easily be modified andapplied to Half-FDD (H-FDD) mode or Time Division Duplex (TDD) mode.

FIG. 2 is a view illustrating structures of a control plane and a userplane of a radio interface protocol between a UE and an E-UTRAN based onthe 3GPP radio access network specification. The control plane refers toa path through which control messages used by a User Equipment (UE) anda network to manage a call are transmitted. The user plane refers to apath through which data generated in an application layer, e.g. voicedata or Internet packet data, is transmitted.

A physical layer of a first layer provides an information transferservice to an upper layer using a physical channel. The physical layeris connected to a Medium Access Control (MAC) layer of an upper layervia a transport channel. Data is transported between the MAC layer andthe physical layer via the transport channel. Data is also transportedbetween a physical layer of a transmitting side and a physical layer ofa receiving side via a physical channel. The physical channel uses timeand frequency as radio resources. Specifically, the physical channel ismodulated using an Orthogonal Frequency Division Multiple Access (OFDMA)scheme in downlink and is modulated using a Single-Carrier FrequencyDivision Multiple Access (SC-FDMA) scheme in uplink.

A MAC layer of a second layer provides a service to a Radio Link Control(RLC) layer of an upper layer via a logical channel. The RLC layer ofthe second layer supports reliable data transmission. The function ofthe RLC layer may be implemented by a functional block within the MAC. APacket Data Convergence Protocol (PDCP) layer of the second layerperforms a header compression function to reduce unnecessary controlinformation for efficient transmission of an Internet Protocol (IP)packet such as an IPv4 or IPv6 packet in a radio interface having arelatively narrow bandwidth.

A Radio Resource Control (RRC) layer located at the bottommost portionof a third layer is defined only in the control plane. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to configuration, re-configuration, and release of radiobearers. The radio bearers refer to a service provided by the secondlayer to transmit data between the UE and the network. To this end, theRRC layer of the UE and the RRC layer of the network exchange RRCmessages. The UE is in an RRC connected mode if an RRC connection hasbeen established between the RRC layer of the radio network and the RRClayer of the UE. Otherwise, the UE is in an RRC idle mode. A Non-AccessStratum (NAS) layer located at an upper level of the RRC layer performsfunctions such as session management and mobility management.

One cell of an eNB is set to use one of bandwidths such as 1.25, 2.5, 5,10, 15, and 20 MHz to provide a downlink or uplink transmission serviceto a plurality of UEs. Different cells may be set to provide differentbandwidths.

Downlink transport channels for data transmission from a network to a UEinclude a Broadcast Channel (BCH) for transmitting system information, aPaging Channel (PCH) for transmitting paging messages, and a downlinkShared Channel (SCH) for transmitting user traffic or control messages.Traffic or control messages of a downlink multicast or broadcast servicemay be transmitted through the downlink SCH or may be transmittedthrough an additional downlink Multicast Channel (MCH). Meanwhile,uplink transport channels for data transmission from the UE to thenetwork include a Random Access Channel (RACH) for transmitting initialcontrol messages and an uplink SCH for transmitting user traffic orcontrol messages. Logical channels, which are located at an upper levelof the transport channels and are mapped to the transport channels,include a Broadcast Control Channel (BCCH), a Paging Control Channel(PCCH), a Common Control Channel (CCCH), a Multicast Control Channel(MCCH), and a Multicast Traffic Channel (MTCH).

FIG. 3 is a view illustrating physical channels used in a 3GPP systemand a general signal transmission method using the same.

A UE performs initial cell search such as establishment ofsynchronization with an eNB when power is turned on or the UE enters anew cell (step S301). The UE may receive a Primary SynchronizationChannel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from theeNB, establish synchronization with the eNB, and acquire informationsuch as a cell identity (ID). Thereafter, the UE may receive a physicalbroadcast channel from the eNB to acquire broadcast information withinthe cell. Meanwhile, the UE may receive a Downlink Reference Signal (DLRS) in the initial cell search step to confirm a downlink channel state.

Upon completion of initial cell search, the UE may receive a PhysicalDownlink Control Channel (PDCCH) and a Physical Downlink Shared Channel(PDSCH) according to information carried on the PDCCH to acquire moredetailed system information (step S302).

Meanwhile, if the UE initially accesses the eNB or if radio resourcesfor signal transmission are not present, the UE may perform a randomaccess procedure (steps S303 to S306) with respect to the eNB. To thisend, the UE may transmit a specific sequence through a Physical RandomAccess Channel (PRACH) as a preamble (steps S303 and S305), and receivea response message to the preamble through the PDCCH and the PDSCHcorresponding thereto (steps S304 and S306). In the case of acontention-based RACH, a contention resolution procedure may beadditionally performed.

The UE which performs the above procedures may receive a PDCCH/PDSCH(step S307) and transmit a Physical Uplink Shared Channel(PUSCH)/Physical Uplink Control Channel (PUCCH) (step S308) according toa general uplink/downlink signal transmission procedure. Especially, theUE receives Downlink Control Information (DCI) through the PDCCH. TheDCI includes control information such as resource allocation informationfor the UE and has different formats according to use purpose.

Meanwhile, control information, transmitted by the UE to the eNB throughuplink or received by the UE from the eNB through downlink, includes adownlink/uplink ACKnowledgment/Negative ACKnowledgment (ACK/NACK)signal, a Channel Quality Indicator (CQI), a Precoding Matrix Index(PMI), a Rank Indicator (RI), and the like. In the case of the 3GPP LTEsystem, the UE may transmit control information such as CQI/PMI/RIthrough the PUSCH and/or the PUCCH.

FIG. 4 is a view illustrating the structure of a radio frame used in anLTE system.

Referring to FIG. 4, the radio frame has a length of 10 ms (327200 Ts)and includes 10 equally-sized subframes. Each of the subframes has alength of 1 ms and includes two slots. Each of the slots has a length of0.5 ms (15360 Ts). In this case, Ts denotes sampling time and isrepresented by Ts=1/(15 kHz×2048)=3.2552×10-8 (about 33 ns). Each slotincludes a plurality of OFDM symbols in a time domain and includes aplurality of Resource Blocks (RBs) in a frequency domain. In the LTEsystem, one resource block includes 12 subcarriers x 7 (or 6) OFDMsymbols. A Transmission Time Interval (TTI), which is a unit time fordata transmission, may be determined in units of one or more subframes.The above-described structure of the radio frame is purely exemplary andvarious modifications may be made in the number of subframes included ina radio frame, the number of slots included in a subframe, or the numberof OFDM symbols included in a slot.

FIG. 5 is a view illustrating control channels contained in a controlregion of one subframe in a downlink radio frame.

Referring to FIG. 5, one subframe includes 14 OFDM symbols. The first tothird ones of the 14 OFDM symbols may be used as a control region andthe remaining 13 to 11 OFDM symbols may be used as a data region,according to subframe configuration. In FIG. 5, R1 to R4 representreference signals (RSs) or pilot signals for antennas 0 to 3,respectively. The RSs are fixed to a predetermined pattern within thesubframe irrespective of the control region and the data region. Controlchannels are allocated to resources to which the RS is not allocated inthe control region. Traffic channels are allocated to resources, towhich the RS is not allocated, in the data region. The control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH, physical control format indicator channel, informs a UE ofthe number of OFDM symbols used for the PDCCH per subframe. The PCFICHis located in the first OFDM symbol and is established prior to thePHICH and the PDCCH. The PCFICH is comprised of 4 Resource ElementGroups (REGs) and each of the REGs is distributed in the control regionbased on a cell ID. One REG includes 4 Resource Elements (REs). The REindicates a minimum physical resource defined as one subcarrier x oneOFDM symbol. The PCFICH value indicates values of 1 to 3 or values of 2to 4 depending on bandwidth and is modulated by Quadrature Phase ShiftKeying (QPSK).

The PHICH, physical Hybrid-ARQ indicator channel, is used to transmit aHARQ ACK/NACK signal for uplink transmission. That is, the PHICHindicates a channel through which downlink ACK/NACK information foruplink HARQ is transmitted. The PHICH includes one REG and iscell-specifically scrambled. The ACK/NACK signal is indicated by 1 bitand is modulated by Binary Phase Shift Keying (BPSK). The modulatedACK/NACK signal is spread by a Spreading Factor (SF)=2 or 4. A pluralityof PHICHs mapped to the same resource constitutes a PHICH group. Thenumber of PHICHs multiplexed to the PHICH group is determined dependingon the number of SFs. The PHICH (group) is repeated three times toobtain diversity gain in a frequency domain and/or a time domain.

The PDCCH, physical downlink control channel, is allocated to the firstn OFDM symbols of a subframe. In this case, n is an integer greater than1 and is indicated by the PCFICH. The PDCCH is comprised of one or moreControl Channel Elements (CCEs). The PDCCH informs each UE or UE groupof information associated with resource allocation of a Paging Channel(PCH) and a Downlink-Shared Channel (DL-SCH), uplink scheduling grant,Hybrid Automatic Repeat Request (HARQ) information, etc. Therefore, aneNB and a UE transmit and receive data other than specific controlinformation or specific service data through the PDSCH.

Information indicating to which UE or UEs PDSCH data is to betransmitted, information indicating how UEs are to receive PDSCH data,and information indicating how UEs are to perform decoding are containedin the PDCCH. For example, it is assumed that a specific PDCCH isCRC-masked with a Radio Network Temporary Identity (RNTI) “A” andinformation about data, that is transmitted using radio resources “B”(e.g., frequency location) and transport format information “C” (e.g.,transmission block size, modulation scheme, coding information, etc.),is transmitted through a specific subframe. In this case, a UE locatedin a cell monitors the PDCCH using its own RNTI information. If one ormore UEs having the RNTI ‘A’ are present, the UEs receive the PDCCH andreceive the PDSCH indicated by ‘B’ and ‘C’ through the received PDCCHinformation.

FIG. 6 illustrates the structure of an uplink subframe used in the LTEsystem.

Referring to FIG. 6, an uplink subframe is divided into a region towhich a PUCCH is allocated to transmit control information and a regionto which a PUSCH is allocated to transmit user data. The PUSCH isallocated to the middle of the subframe, whereas the PUCCH is allocatedto both ends of a data region in the frequency domain. The controlinformation transmitted on the PUCCH includes an ACK/NACK, a CQIrepresenting a downlink channel state, an RI for Multiple Input andMultiple Output (MIMO), a Scheduling Request (SR) indicating a requestfor allocation of uplink resources, etc. A PUCCH of a UE occupies one RBin a different frequency in each slot of a subframe. That is, two RBsallocated to the PUCCH frequency-hop over the slot boundary.Particularly, FIG. 6 illustrates an example in which PUCCHs for m=0,m=1, m=2, and m=3 are allocated to a subframe.

MIMO System

Hereinafter, a MIMO system will be described. MIMO refers to a method ofusing multiple transmission antennas and multiple reception antennas toimprove data transmission/reception efficiency. Namely, a plurality ofantennas is used at a transmitting end or a receiving end of a wirelesscommunication system so that capacity can be increased and performancecan be improved. MIMO may also be referred to as ‘multi-antenna’ in thisdisclosure.

MIMO technology does not depend on a single antenna path in order toreceive a whole message. Instead, MIMO technology collects datafragments received via several antennas, merges the data fragments, andforms complete data. The use of MIMO technology can increase systemcoverage while improving data transfer rate within a cell area of aspecific size or guaranteeing a specific data transfer rate. MIMOtechnology can be widely used in mobile communication terminals andrelay nodes. MIMO technology can overcome the limitations of therestricted amount of transmission data of single antenna based mobilecommunication systems.

The configuration of a general MIMO communication system is shown inFIG. 7. A transmitting end is equipped with NT transmission (Tx)antennas and a receiving end is equipped with NR reception (Rx)antennas. If a plurality of antennas is used both at the transmittingend and at the receiving end, theoretical channel transmission capacityincreases unlike the case where only either the transmitting end or thereceiving end uses a plurality of antennas. Increase in channeltransmission capacity is proportional to the number of antennas, therebyimproving transfer rate and frequency efficiency. If a maximum transferrate using a signal antenna is Ro, a transfer rate using multipleantennas can be theoretically increased by the product of the maximumtransfer rate Ro by a rate increment Ri. The rate increment Ri isrepresented by the following equation 1 where Ri is the smaller of NTand NR.

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, in a MIMO communication system using four Tx antennas andfour Rx antennas, it is possible to theoretically acquire a transferrate four times that of a single antenna system. After theoreticalincrease in the capacity of the MIMO system was first demonstrated inthe mid-1990s, various techniques for substantially improving datatransfer rate have been under development. Several of these techniqueshave already been incorporated into a variety of wireless communicationstandards including, for example, 3rd generation mobile communicationand next-generation wireless local area networks.

Active research up to now related to MIMO technology has focused upon anumber of different aspects, including research into information theoryrelated to MIMO communication capacity calculation in various channelenvironments and in multiple access environments, research into wirelesschannel measurement and model derivation of MIMO systems, and researchinto space-time signal processing technologies for improvingtransmission reliability and transfer rate.

To describe a communication method in a MIMO system in detail, amathematical model thereof is given below. As shown in FIG. 7, it isassumed that NT Tx antennas and NR Rx antennas are present. In the caseof a transmission signal, a maximum number of transmittable pieces ofinformation is NT under the condition that NT Tx antennas are used, sothat transmission information can be represented by a vector representedby the following equation 2:

S=└s ₁ ,s ₂ ,Λ,s _(N) _(T) ┘^(T)

Meanwhile, individual transmission information pieces s₁, s₂, . . . ,s_(N) _(T) may have different transmission powers. In this case, if theindividual transmission powers are denoted by P₁, P₂, . . . , P_(N) _(T), transmission information having adjusted transmission powers can berepresented by a vector shown in the following equation 3:

ŝ=[ŝ₁ ,ŝ ₂ ,Λ,ŝ _(N) _(T) ]^(T)=[P ₁ s ₁ ,P ₂ s ₂ ,Λ,P _(N) _(T) ,s _(N)_(T) ]^(T)

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of a transmissionpower:

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

NT transmission signals x₁, x₂, . . . x_(N) _(T) to be actuallytransmitted may be configured by multiplying the transmissionpower-controlled information vector Ŝ by a weight matrix W. In thiscase, the weight matrix is adapted to properly distribute transmissioninformation to individual antennas according to transmission channelsituations. The transmission signals x₁, x₂, . . . x_(N) _(T) can berepresented by the following Equation 5 using a vector X. In Equation 5,W_(ij) is a weight between the i-th Tx antenna and the j-th informationand W is a weight matrix, which may also be referred to as a precodingmatrix.

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\M \\x_{i} \\M \\x_{N_{T}}\end{bmatrix} = {\quad{{\begin{bmatrix}w_{11} & w_{12} & \Lambda & w_{1\; N_{T}} \\w_{21} & w_{22} & \Lambda & w_{2\; N_{T}} \\M & \; & O & \; \\w_{i\; 1} & w_{i\; 2} & \Lambda & w_{i\; N_{T}} \\M & \; & O & \; \\w_{N_{T}1} & w_{N_{T}2} & \Lambda & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\M \\{\hat{s}}_{j} \\M \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\; \hat{s}} = {WPs}}}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Generally, the physical meaning of a rank of a channel matrix may be amaximum number of different pieces of information that can betransmitted in a given channel. Accordingly, since the rank of thechannel matrix is defined as the smaller of the number of rows orcolumns, which are independent of each other, the rank of the matrix isnot greater than the number of rows or columns. A rank of a channelmatrix H, rank(H), is restricted as follows.

rank(H)≤min(N _(T) ,N _(R))  [Equation 6]

Each unit of different information transmitted using MIMO technology isdefined as a ‘transmission stream’ or simply ‘stream’. The ‘stream’ maybe referred to as a ‘layer’. The number of transmission streams is notgreater than a rank of a channel which is a maximum number of differentpieces of transmittable information. Accordingly, the channel matrix Hmay be indicted by the following Equation 7:

# of streams≤rank(H)≤min(N _(T) ,N _(R))  [Equation 7]

where ‘# of streams’ denotes the number of streams. It should be notedthat one stream may be transmitted through one or more antennas.

There may be various methods of allowing one or more streams tocorrespond to multiple antennas. These methods may be described asfollows according to types of MIMO technology. The case where one streamis transmitted via multiple antennas may be called spatial diversity,and the case where multiple streams are transmitted via multipleantennas may be called spatial multiplexing. It is also possible toconfigure a hybrid of spatial diversity and spatial multiplexing.

CSI Feedback

Now, a description of a Channel State Information (CSI) report is given.In the current LTE standard, a MIMO transmission scheme is categorizedinto open-loop MIMO operated without CSI and closed-loop MIMO operatedbased on CSI. Especially, according to the closed-loop MIMO system, eachof the eNB and the UE may be able to perform beamforming based on CSI toobtain a multiplexing gain of MIMO antennas. To obtain CSI from the UE,the eNB allocates a PUCCH or a PUSCH to command the UE to feed back CSIfor a downlink signal.

CSI is divided into three types of information: a Rank Indicator (RI), aPrecoding Matrix Index (PMI), and a Channel Quality Indicator (CQI).First, RI is information on a channel rank as described above andindicates the number of streams that can be received via the sametime-frequency resource. Since RI is determined by long-term fading of achannel, it may be generally fed back at a cycle longer than that of PMIor CQI.

Second, PMI is a value reflecting a spatial characteristic of a channeland indicates a precoding matrix index of the eNB preferred by the UEbased on a metric of Signal-to-Interference plus Noise Ratio (SINR).Lastly, CQI is information indicating the strength of a channel andindicates a reception SINR obtainable when the eNB uses PMI.

In an evolved communication system such as an LTE-A system, multi-userdiversity using Multi-User MIMO (MU-MIMO) is additionally obtained.Since interference between UEs multiplexed in an antenna domain existsin the MU-MIMO scheme, CSI accuracy may greatly affect not onlyinterference of a UE that has reported CSI but also interference ofother multiplexed UEs. Hence, in order to correctly perform MU-MIMOoperation, it is necessary to report CSI having accuracy higher thanthat of a Single User-MIMO (SU-MIMO) scheme.

Accordingly, LTE-A standard has determined that a final PMI should beseparately designed into W1, which a long-term and/or wideband PMI, andW2, which is a short-term and/or subband PMI.

An example of a hierarchical codebook transform scheme configuring onefinal PMI from among W1 and W2 may use a long-term covariance matrix ofa channel as indicated in Equation 8:

W=norm(W1W2)  [Equation 8]

In Equation 8, W2 of a short-term PMI indicates a codeword of a codebookconfigured to reflect short-term channel information, W denotes acodeword of a final codebook, and norm(A) indicates a matrix in which anorm of each column of a matrix A is normalized to 1.

The detailed configurations of W1 and W2 are shown in Equation 9:

$\begin{matrix}{{{W\; 1(i)} = \begin{bmatrix}X_{i} & 0 \\0 & X_{i}\end{bmatrix}},{{{where}\mspace{14mu} X_{i}\mspace{14mu} {is}\mspace{14mu} {{Nt}/2}\mspace{14mu} {by}\mspace{14mu} M\mspace{14mu} {{matrix}.W}\; 2(j)} = {\overset{\overset{r\mspace{14mu} {columns}}{}}{\begin{bmatrix}e_{M}^{k} & e_{M}^{l} & \ldots & e_{M}^{m} \\{\alpha_{j}e_{M}^{k}} & {\beta_{j}e_{M}^{l}} & \; & {\gamma_{j}e_{M}^{m}}\end{bmatrix}}\mspace{14mu} \left( {{{if}\mspace{14mu} {rank}} = r} \right)}},{{{where}\mspace{14mu} 1} \leq k},l,{m \leq {M\mspace{14mu} {and}\mspace{14mu} k}},l,{m\mspace{14mu} {are}\mspace{14mu} {{integer}.}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

where Nt is the number of Tx antennas, M is the number of columns of amatrix Xi, indicating that the matrix Xi includes a total of M candidatecolumn vectors. eMk, eMl, and eMm denote k-th, I-th, and m-th columnvectors of the matrix Xi in which only k-th, I-th, and m-th elementsamong M elements are 0 and the other elements are 0, respectively.α_(j), β_(j), and γ_(j) are complex values each having a unit norm andindicate that, when the k-th, I-th, and m-th column vectors of thematrix Xi are selected, phase rotation is applied to the column vectors.At this time, i is an integer greater than 0, denoting a PMI indexindicating W1 and j is an integer greater than 0, denoting a PMI indexindicating W2.

In Equation 9, the codebook configurations are designed to reflectchannel correlation properties generated when cross polarized antennasare used and when a space between antennas is dense, for example, when adistance between adjacent antennas is less than a half of signalwavelength. The cross polarized antennas may be categorized into ahorizontal antenna group and a vertical antenna group. Each antennagroup has the characteristic of a Uniform Linear Array (ULA) antenna andthe two groups are co-located.

Accordingly, a correlation between antennas of each group hascharacteristics of the same linear phase increment and a correlationbetween antenna groups has characteristics of phase rotation.Consequently, since a codebook is a value obtained by quantizing achannel, it is necessary to design a codebook such that characteristicsof a channel are reflected. For convenience of description, a rank-1codeword generated by the aforementioned configurations is shown asfollows:

$\begin{matrix}{{W\; 1(i)*W\; 2(j)} = \begin{bmatrix}{X_{i}(k)} \\{\alpha_{j}{X_{i}(k)}}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

In Equation 10, a codeword is expressed as a vector of N_(T)×1 (where NTis the number of Tx antennas) and is structured with an upper vectorX_(i)(k) and a lower vector α_(j)X_(i)(k) which show correlationcharacteristics of a horizontal antenna group and a vertical antennagroup, respectively. X_(i)(k) is preferably expressed as a vector havingthe characteristics of linear phase increment by reflecting thecharacteristics of a correlation between antennas of each antenna groupand may be a DFT matrix as a representative example.

As described above, CSI in the LTE system includes, but is not limitedto, CQI, PMI, and RI. According to transmission mode of each UE, all orsome of the CQI, PMI, and RI is transmitted. Periodic transmission ofCSI is referred to as periodic reporting and transmission of CSI at therequest of an eNB is referred to as aperiodic reporting. In aperiodicreporting, a request bit included in uplink scheduling informationtransmitted by the eNB is transmitted to the UE. Then, the UE transmitsCSI considering transmission mode thereof to the eNB through an uplinkdata channel (PUSCH). In periodic reporting, a period of CSI and anoffset at the period are signaled in the unit of subframes by asemi-static scheme through a higher-layer signal per UE. The UEtransmits CSI considering transmission mode to the eNB through an uplinkcontrol channel (PUCCH). If there is uplink data in a subframe in whichCSI is transmitted, the CSI is transmitted through an uplink datachannel (PUSCH) together with the uplink data. The eNB transmitstransmission timing information suitable for each UE to the UE inconsideration of a channel state of each UE and a UE distributedsituation in a cell. The transmission timing information includes aperiod and an offset necessary for transmitting CSI and may betransmitted to each UE through an RRC message.

FIGS. 8 to 11 illustrate periodic reporting of CSI in an LTE system.

Referring to FIG. 8, there are four CQI reporting modes in the LTEsystem. Specifically, the CQI reporting modes may be divided into modesin a WideBand (WB) CQI and modes in a SubBand (SB) CQI according to CQIfeedback type. The CQI reporting mode may also be divided into modes ina No PMI and modes in a single PMI depending on whether a PMI istransmitted or not. Each UE is informed of information comprised of aperiod and an offset through RRC signaling in order to periodicallyreport CQI.

FIG. 9 illustrates an example of transmitting CSI when a UE receivesinformation indicating (a period ‘5’ and an offset ‘1’) throughsignaling. Referring to FIG. 9, upon receiving the informationindicating the period ‘5’ and offset ‘1’, the UE transmits CSI in theunit of 5 subframes with an offset of one subframe in ascending order ofa subframe index counted from 0 starting from the first subframe.Although the CSI is transmitted basically through a PUCCH, if a PUSCHfor data transmission is present at the same transmission time point,the CSI is transmitted through the PUSCH together with data. Thesubframe index is given as a combination of a system frame number (or aradio frame index) nf and a slot index ns (0 to 19). Since one subframeincludes two slots, the subframe index may be defined as10×nf+floor(ns/2) wherein floor( ) indicates the floor function.

CQI transmission types include a type of transmitting a WB CQI only anda type of transmitting both a WB CQI and an SB CQI. In the type oftransmitting a WB CQI only, CQI information for all bands is transmittedin subframes corresponding to every CQI transmission period. Meanwhile,in the case in which PMI information should also be transmittedaccording to the PMI feedback type as illustrated in FIG. 8, the PMIinformation is transmitted together with the CQI information. In thetype of transmitting both a WB CQI and an SB CQI, the WB CQI and SB CQIare alternately transmitted.

FIG. 10 illustrates a system in which a system bandwidth consists of 16RBs. It is assumed that the system bandwidth includes two BandwidthParts (BPs) BP0 and BP1 each consisting of two SubBands (SBs) SB0 andSB1 and each SB includes 4 RBs. The above assumption is exemplary andthe number of BPs and the size of each SB may vary with the size of thesystem bandwidth. The number of SBs constituting each BP may differaccording to the number of RBs, the number of BPs, and the size of eachSB.

In the CQI transmission type of transmitting both a WB CQI and an SBCQI, the WB CQI is transmitted in the first CQI transmission subframeand an SB CQI of the better SB state of SB0 and SB1 in BP0 istransmitted in the next CQI transmission subframe together with and anindex of the corresponding SB (e.g. Subband Selection Indicator (SSI).Thereafter, an SB CQI of the better SB state of SB0 and SB1 in BP1 andan index of the corresponding SB are transmitted in the next CQItransmission subframe. Thus, CQI of each BP is sequentially transmittedafter transmission of the WB CQI. The CQI of each BP may be sequentiallytransmitted once to four times during the interval between transmissionintervals of two WB CQIs. For example, if the CQI of each BP istransmitted once during the time interval between two WB CQIs, CQIs maybe transmitted in the order of WB CQI ⇒BP0 CQI⇒BP1 CQI⇒WB CQI. If theCQI of each BP is transmitted four times during the time intervalbetween two WB CQIs, CQIs may be transmitted in the order of WB CQI ⇒BP0CQI ⇒BP1 CQI⇒BP0 CQI⇒BP1 CQI⇒BP0 CQI⇒BP1 CQI⇒BP0 CQI⇒BP1 CQI⇒WB CQI.Information as to how many times each BP CQI is transmitted is signaledby a higher layer (RRC layer).

FIG. 11(a) illustrates an example of transmitting both a WB CQI and anSB CQI when a UE receives information indicating (period ‘5’ and offset‘1’) through signaling. Referring to FIG. 11(a), a CQI may betransmitted only in subframes corresponding to the signaled period andoffset regardless of type.

FIG. 11(b) illustrates an example of transmitting an RI in addition tothe example shown in FIG. 11(a). The RI may be signaled as a combinationof a multiple of a WB CQI transmission period and an offset at thetransmission period from a higher layer (e.g. RRC layer). The offset ofthe RI is signaled using a value relative to the offset of a CQI. Forexample, if the offset of the CQI is ‘1’ and the offset of the RI is‘0’, the RI has the same offset as the CQI. The offset value of the RIis defined as 0 or a negative number. More specifically, it is assumedin FIG. 11(b) that, in an environment identical to that of FIG. 11(a),an RI transmission period is a multiple of 1 of the WB CQI transmissionperiod and the RI offset is ‘−1’. Since the RS transmission period is amultiple of 1 of the WB CQI transmission period, the RS transmissionperiod and the WB CQI transmission period are substantially the same.Since the offset of the RI is ‘−1’, the RI is transmitted based upon thevalue ‘−1’ (i.e. subframe index 0) relative to the offset ‘1’ of the CQIin FIG. 11(a). If the offset of the RI is ‘0’, the transmissionsubframes of the WB CQI and RI overlap. In this case, the WB CQI isdropped and the RI is transmitted.

FIG. 12 is a diagram illustrating periodic reporting of channel stateinformation discussed in an LTE-A system. When an eNB has 8 transmitantennas, in the case of mode 2-1, a precoder type indication (PTI)parameter as a 1-bit indicator may be set, and a periodic reporting modesubdivided into two types may be considered according to the PTI value,as shown in FIG. 12. In the drawing, W1 and W2 indicate a hierarchicalcodebook described with reference to Equations 8 and 9 above. When bothW1 and W2 are determined, W1 and W2 are combined to determine a completeform of precoding matrix W.

Referring to FIG. 12, in the case of periodic reporting, differentinformation items corresponding to Report 1, Report 2, and Report 3 arereported with different repetition periodicities. Report 1 reports R1and a 1-bit PTI value. Report 2 reports wideband (WB) W1 (in the case ofPTI=0) or WB W2 and WB CQI (in the case of PTI=1). Report 3 reports WBW2 and WB CQI (in the case of PTI=0) or subband (SB) W2 and SB CQI (inthe case of PTI=1).

In Report 2 and Report 3, a subframe index is transmitted in a subframe(for convenience, referred to as a first subframe set) that satisfies(10*nf+floor(ns/2)−N offset, CQI) mod (Nc)=0. N offset, CQIcorresponding to an offset value for PMI/CQI transmission illustrated inFIG. 9. In addition, Nc indicates a subframe interval between adjacentReport 2 or Report 3. FIG. 12 illustrates the case of N offset, CQI=1and Nc=2, and the first subframe set is configured with subframes withan odd index. nf indicates a system frame number (or a radio frameindex), and ns indicates a slot index in a radio frame. floor( )indicates a rounddown function, and A mod B indicates a remainderobtained by dividing A by B.

Report 2 is positioned on some subframes in the first subframe set, andReport 3 is positioned on the remaining subframes. In detail, Report 2is positioned on a subframe in which a subframe index satisfies(10*nf+floor(ns/2)−N offset, CQI) mod (H*Nc)=0. Accordingly, Report 2 istransmitted every interval of H*Nc, and Report 3 transmission is filledin one or more first subframes between adjacent Report 2. In the case ofPTI=0, H=M and M is determined via high layer signaling. FIG. 12illustrates the case of M=2. When PTI=1, H=J*K+1, K is determined viahigh layer signaling, and J is the number of bandwidth parts (BPs). FIG.12 illustrates the case of J=3 and K=1.

Report 1 is transmitted in a subframe in which a subframe indexsatisfies (10*nf+floor(ns/2)−N offset, CQI-N offset, RI) mod(MRI*(J*K+1)*Nc)=0, and MRI is determined via high layer signaling. Noffset, RI indicates a relative offset value for RI, and FIG. 12illustrates the case of MRI=2 and N offset, RI=−1. According to Noffset, RI=−1, transmission time points of Report 1 and Report 2 are notoverlapped with each other. When a UE calculates R1, W1, and W2, RI, W1,and W2 are associated and calculated. For example, W1 and W2 arecalculated according to RI, and W2 is calculated according to W1. Whenboth Report 2 and Report 3 are reported subsequent to Report 1, an eNBmay know final W from W1 and W2.

FIG. 13 is a diagram illustrating CSI feedback in submode 1 of mode 1-1of FIG. 8.

When PUCCH feedback mode 1-1 uses a dual codebook, submode 1 and submode2 are present. FIG. 13 illustrates submode 1. Wideband W2 and widebandCQI are set to offset 1 and periodicity 2 and RI and W1 are set tooffset 0 and periodicity 16.

In the 8Tx codebook, as shown in Table 1 below, RI and W1 arejoint-encoded in 5 bits and in this case, and W1 is subsampled as shownin Table 1 below in order to reduce the sizes of payloads of RI and W1to report information with a low coding rate. Since RI is referred to bythe remaining PMI and CQI, encoding needs to be performed with a lowcoding rate in order to prevent a decoding error in RI from occurring.

TABLE 1 hypotheses RI values  0-7 1 {0, 2, 4, 6, 8, 10, 12, 14}  8-15 2{0, 2, 4, 6, 8, 10, 12, 14} 16-17 3 {0, 2} 18-19 4 {0, 2} 20-21 5 {0, 2}22-23 6 {0, 2} 24-25 7 {0, 2} 26 8 {0} 27-31 reserved NA

FIG. 14 is a diagram illustrating CSI feedback in submode 2 of mode 1-1of FIG. 8.

As described above, when PUCCH feedback mode 1-1 uses a dual codebookstructure, submode 1 and submode 2 are present. FIG. 14 illustrates anexample of submode 2. Wideband W1/W2 and wideband CQI are set withoffset 1 and periodicity 2. RI is set with offset 0 and periodicity 16.

CSI information to an eNB through PUCCH format 2. That is, CSIinformation may be transmitted in 11 bits as a payload size of PUCCHformat 2. Accordingly, a codebook needs to be subsampled such that apayload of type 2 c does not exceed a total of 11 bits. To this end, in8Tx codebook, W1 and W2 are subsampled to report type 2 c, as shown inTable 2 below.

TABLE 2 PMI for W1 PMI for W2 total RI #bits values #bits values #bits 13 {0, 2, 4, 6, 8, 10, 12, 14} 1 {0, 2} 4 2 3 {0, 2, 4, 6, 8, 10, 12, 14}1 {0, 1} 4 3 1 {0, 2} 3 {0, 1, 2, 3, 8, 9, 4 10, 11} 4 1 {0, 1} 3 {0, 1,2, 3, 4, 5, 4 6, 7} 5 2 {0, 1, 2, 3} 0 {0} 2 6 2 {0, 1, 2, 3} 0 {0} 2 72 {0, 1, 2, 3} 0 {0} 2 8 0 {0} 0 {0} 0

8Tx W1 for rank 1 and 8Tx W1 for rank 2 are the same. In addition, ithPMI and (i+1)th PMI of W 1 share two overlapped DFT vectors. As such,two DFT vectors may be overlapped between adjacent PMIs, thereby moreaccurately feeding back a channel. However, due to a limited PUCCHresource, PMI of even-numbered W1 may be limited to an even number andsubsampled as shown in Table 2 above. Overlapped DFT vectors betweeneven-numbered PMIs are not present, but the UE can represent a total of32 DFT vectors using the subsampled W1, thereby minimizing performancedegradation.

FIG. 15 is a diagram illustrating CSI feedback in mode 2-1 of FIG. 8.

When PUCCH feedback mode 2-1 uses a dual codebook structure, two methodsare defined according to a PTI value. FIG. 15(a) illustrates the case inwhich PTI is 0 and FIG. 15(b) illustrates the case in which PTI is 1.Referring to FIG. 15(a), wideband W1 is reported with periodicity of 8subframes in a PUCCH feedback resource with offset 1 and periodicity 2,and wideband W2 and CQI are reported in the remaining resource. RI andPTI are set with periodicity 16 and offset 0. In FIG. 15(b), when PTI isset to 1, subband W2 and subband CQI and L-bit information indicating asubband index are reported.

In FIG. 15(b), in type 1 a report in which subband W2 and subband CQIand L-bit information indicating a subband index are reported together,8Tx codebook W2 is subsampled as shown in Table 3 below. Information in11 bits as a payload size of PUCCH format 2 may be transmitted throughthe subsampling. In Table 2, W2 codeword of rank 2 reports only 0, 2, 4,and 6. These values perform a function for selecting one beam from beamgroups constituting W1 to general a final codebook. For example, when W1is configured according to the following equation, if codeword 0 of W2is selected, final codebook W is determined as

$W = \begin{bmatrix}w_{11} & w_{11} \\w_{11} & {- w_{11}}\end{bmatrix}$

using only w₁₁. In Equation 11 below, w11, w12, w13, and w14 indicate a4×1 column vector.

$\begin{matrix}{{W\; 1} = \begin{bmatrix}\begin{bmatrix}w_{11} & w_{12} & w_{13} & w_{14}\end{bmatrix} & 0 \\0 & \begin{bmatrix}w_{11} & w_{12} & w_{13} & w_{14}\end{bmatrix}\end{bmatrix}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Similarly, when codeword 2 of W2 is selected, the final codebook W isdetermined using only w₁₂, when codeword 4 of W2 is selected, the finalcodebook W is determined using only w₁₃, and when codeword 6 of W2 isselected, the final codebook W is determined using only w₁₄.

Table 3 below shows codebook subsampling in PUCCH mode 2-1. modindicates modular operation.

TABLE 3 Relationship between the second PMI value and codebook index i₂Value of the second PMI RI I_(PMI2) Codebook index i₂ 1 0-15 I_(PMI2) 20-3 2I_(PMI2) 3 0-3 8 · └I_(PMI2)/2┘ + (I_(PMI2) mod2) + 2 4 0-32I_(PMI2) 5 0 0 6 0 0 7 0 0 8 0 0

A CSI reporting type may be set to one of various types. For example, aCSI reporting type defined in LTE release-10 will now be described. Type1 reporting supports CQI feedback for UE selection sub-bands. Type 1 areporting supports subband CQI and second PMI feedback. Type 2, Type 2b, and Type 2 c reporting supports wideband CQI and PMI feedback. Type 2a reporting supports wideband PMI feedback. Type 3 reporting supports RIfeedback. Type 4 reporting supports wideband CQI. Type 5 reportingsupports RI and wideband PMI feedback. Type 6 reporting supports RI andPTI feedback.

4Tx Codebook

4 Tx codebook may be represented by multiplication of two matrices asfollows.

W=W ₁ ·W ₂  [Equation 12]

Here, the inner precoder W₁ and the outer precoder W₂ may representwideband/long-term channel properties and subband/short-term channelproperties, respectively. W₁ may be set as follows.

$\begin{matrix}{{W_{1} = \begin{bmatrix}X_{n} & 0 \\0 & X_{n}\end{bmatrix}},{n = 0},1,\Lambda,15} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack\end{matrix}$

Here, X_(n) may be set as follows.

$\begin{matrix}{X_{n} = {{\begin{bmatrix}1 & 1 & 1 & 1 \\q_{1}^{n} & q_{1}^{n + 8} & q_{1}^{n + 16} & q_{1}^{n + 24}\end{bmatrix}\mspace{14mu} {where}\mspace{14mu} q_{1}} = e^{j\; 2\; {\pi/32}}}} & \left\lbrack {{Equation}\mspace{14mu} 53} \right\rbrack\end{matrix}$

Codebook W₂ for rank 1 may be set as follows.

$\begin{matrix}{{W_{2,n} \in \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{\alpha (i)}Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{j\; {\alpha (i)}Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{- {\alpha (i)}}\; Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{- j}\; {\alpha (i)}\; Y}\end{bmatrix}}} \right\}},\mspace{79mu} {Y = {{e_{i} \in {\left\{ {e_{1},e_{2},e_{3},e_{4}} \right\} \mspace{14mu} {and}\mspace{14mu} {\alpha (i)}}} = q_{1}^{2{({i - 1})}}}}} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack\end{matrix}$

Codebook W₂ for rank 2 may be set as follows.

$\begin{matrix}{\mspace{79mu} {{W_{2,n} \in \left\{ {{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\{j\; Y_{1}} & {{- j}\; Y_{2}}\end{bmatrix}}} \right\}},{\left( {Y_{1},Y_{2}} \right) = {\left( {e_{i},e_{k}} \right) \in \left\{ {\left( {e_{1},e_{1}} \right),\left( {e_{2},e_{2}} \right),\left( {e_{3},e_{3}} \right),\left( {e_{4},e_{4}} \right),\left( {e_{1},e_{2}} \right),\left( {e_{2},e_{3}} \right),\left( {e_{1},e_{4}} \right),\left( {e_{2},e_{4}} \right)} \right\}}}}} & \left\lbrack {{Equation}\mspace{14mu} 15} \right\rbrack\end{matrix}$

Here, e_(n) is a 4-element selection vector with all zeros except forthe nth element with 1.

In Equation 14, W2 is configured by vertically concatenating two Yvectors, and the lower Y vectors is multiplied by one of 1, −1, j, and−j to compensate for phase rotation between a horizontal beam group anda vertical beam group in an X-pol antenna. In general, 1, −1, j, and −jare referred to as a co-phasor factor. Similarly, in Equation 15, (1,−1) and (j, −j) are considered as a co-phasor factor.

Hereinafter, an index of W is defined as i1, and i1 is the same as indexn of W1 in the aforementioned equation of 4Tx codebook.

An index of W2 is defined as shown in the following table.

TABLE 4 Index of W2 W2 for rank 1 W2 for rank 2  0$\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{q_{1}^{0}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{1} \\e_{1} & {- e_{1}}\end{bmatrix}$  1 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{jq}_{1}^{0}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{1} \\{je}_{1} & {- {je}_{1}}\end{bmatrix}$  2 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{- q_{1}^{0}}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{2} \\e_{2} & {- e_{2}}\end{bmatrix}$  3 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{- {jq}_{1}^{0}}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{2} \\{je}_{2} & {- {je}_{2}}\end{bmatrix}$  4 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{q_{1}^{2}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{3} & e_{3} \\e_{3} & {- e_{3}}\end{bmatrix}$  5 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{jq}_{1}^{2}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{3} & e_{3} \\{je}_{3} & {- {je}_{3}}\end{bmatrix}$  6 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{- q_{1}^{2}}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{4} & e_{4} \\e_{4} & {- e_{4}}\end{bmatrix}$  7 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{- {jq}_{1}^{2}}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{4} & e_{4} \\{je}_{4} & {- {je}_{4}}\end{bmatrix}$  8 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{q_{1}^{4}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{2} \\e_{1} & {- e_{2}}\end{bmatrix}$  9 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{jq}_{1}^{4}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{2} \\{je}_{1} & {- {je}_{2}}\end{bmatrix}$ 10 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{- q_{1}^{4}}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{3} \\e_{2} & {- e_{3}}\end{bmatrix}$ 11 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{- {jq}_{1}^{4}}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{3} \\{je}_{2} & {- {je}_{3}}\end{bmatrix}$ 12 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{q_{1}^{6}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{4} \\e_{1} & {- e_{4}}\end{bmatrix}$ 13 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{jq}_{1}^{6}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{4} \\{je}_{1} & {- {je}_{4}}\end{bmatrix}$ 14 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{- q_{1}^{6}}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\e_{2} & {- e_{4}}\end{bmatrix}$ 15 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{- {jq}_{1}^{6}}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\{je}_{2} & {- {je}_{4}}\end{bmatrix}$

First Embodiment

A first embodiment of the present invention relates to a codebooksubsampling method in submode 1 of PUCCH feedback mode 1-1.

According to the first embodiment of the present invention, a jointencoding method of RI and subsampled W1 in type 5 report may be appliedas shown in Table 5 below. A total of 17 hypotheses are present and thuscan be represented in 5 bits, and an index of RI and W1 for eachhypothesis is shown in Table 5 below. For example, 0th hypothesis refersto RI=1 and W1 index 0 and first hypothesis refers to RI=1 and W1index 1. W1 codebook for ranks 3 and 4 is an identity matrix, and thusseparate signaling for W1 is not required. In Table 5 below, the case inwhich RI is 2 may be represented by a value obtained by subtracting 8from hypotheses.

TABLE 5 hypotheses RI Index of W1 0-7 1 {0, 1, 2, 3, 4, 5, 6, 7} 8-15 2{0, 1, 2, 3, 4, 5, 6, 7} 16 3 None (W1 is identity matrix) 17 4 none (W1is identity matrix)

Similarly to 8Tx W1 codebook, a codebook of 4Tx W1 codebook proposed inEquation 13 has some overlapped values. For example, comparing 0th W1codeword and 8th W1 codeword, Xn of each codeword is configured with thesame vector set. For example, Xn of the 0th W1 codeword is

$X_{0} = \begin{bmatrix}1 & 1 & 1 & 1 \\q_{1}^{0} & q_{1}^{8} & q_{1}^{16} & q_{1}^{24}\end{bmatrix}$

and Xn of the 8th W1 codeword is

${X_{8} = \begin{bmatrix}1 & 1 & 1 & 1 \\q_{1}^{8} & q_{1}^{16} & q_{1}^{24} & q_{1}^{0}\end{bmatrix}},$

and thus it is seen that Xn of each codeword is configured with the samecolumn vector. This feature is the same as in the case of ith W1codeword and (i+8)th W1 codeword. Accordingly, when W1 is subsampled in3 bits, it is effective to remove the overlapped W1. In the subsamplingmethod of Table 5, only codewords from 0 to 7 are subsampled so as notto overlap W1 in consideration of this feature.

It may be possible to perform subsampling using only 8th to 15thcodewords instead of 0th to 7th in Table 5 using the same principle. Dueto the same subsampling principle, only an index is different butcodebook performance is not changed.

As another method, a 4-bit payload may be transmitted in type 5 reportso as to enhance reception decoding probability. In this case, a jointencoding method of RI and the subsampled W1 may be applied to Table 6below. A total of 9 hypotheses are present and thus can be representedin 4 bits, and an index of RI and W for each hypothesis is shown inTable 6 below. For example, 0th hypothesis refers to RI=1 and W1 index 0and first hypothesis refers to RI=I and W1 index 2. W1 codebook forranks 3 and 4 is an identity matrix, and thus separate signaling for W1is not required.

TABLE 6 hypotheses RI Index of W1 0-3 1 {0, 2, 4, 6} 4-7 2 {0, 2, 4, 6}8 3 None (W1 is identity matrix) 9 4 none (W1 is identity matrix)

A subsampling method of Table 6 can be described in terms of two steps.First, like in Table 5, overlapped W1 codewords are removed. Thensubsampling is performed so as to distribute values included in a secondrow of Xn from the remaining (0,1,2,3,4,5,6,7) with an equivalentinterval in (q₁)^(k)=e^(j2πk/32), where k=0,1,2, . . . 31 As such,subsampling is performed so as to distribute the values with anequivalent interval in (q₁)^(k), thereby preventing beams of W1 frombeing concentrated on a specific direction on a codebook space.Accordingly, codebook performance degradation caused by subsampling maybe reduced.

It may be possible to perform subsampling using only (1,3,5,7) codewordinstead of (0,2,4,6) in Table 6 using the same principle. Due to thesame subsampling principle, only an index is different but codebookperformance is not changed.

In Table 5, W1 indexes in ranks 1 and 2 are the same. Similarly, inTable 6, W1 indexes in ranks 1 and 2 are the same. In addition, thesubsampling methods of Tables 5 and 6 may be mixed and configured. Forexample, W1 of rank 1 may use values of Table 5 and W1 of rank 2 may usevalues of Table 6. In this case, ranks 1 and 2 have 8 and 4 hypotheses,respectively, and transmission of type 5 report may be possible using 4bits.

Second Embodiment

A second embodiment of the present invention relates to a codebooksubsampling method in submode 2 of PUCCH feedback mode 1-1.

According to the second embodiment of the present invention, asubsampling method of W1/W2 in type 2 c report may be applied as shownin Table 7 below. For example, only one of {0,1,2,3,4,5,6,7} may bereported as W1 index in rank 1 and only one of {0,2} may be reported asW2 index. W1 codebook for ranks 3 and 4 is an identity matrix, and thusseparate signaling for W1 is not required.

TABLE 7 PMI for W1 PMI for W2 total RI #bits values #bits values #bits 13 {0, 1, 2, 3, 4, 5, 6, 7} 1 {0, 2} 4 2 3 {0, 1, 2, 3, 4, 5, 6, 7} 1 {0,1} 4 3 0 None (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4 identity matrix) 9,10, 11, 12, 13, 14, 15} 4 0 none (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4identity matrix) 9, 10, 11, 12, 13, 14, 15}

In Table 7, W1 is subsampled using the same method as in Table 5 above.The subsampling method of W2 is the same as in the case of 8Tx. W2 issubsampled as shown in Table 7 such that a selection vector of W2 may befixed to e1 and only a co-phasor factor of W2 may be selected. In thiscase, even if W1 is configured with {0,1,2,3,4,5,6,7}, the selectionvector of W2 is fixed to only e1. Accordingly, a final precoding matrixgenerated by multiplying W1 and W2 uses only a front 0th vector to 7thvector among 32 vectors of a 2×32 DFT matrix oversampled 16 times.

When subsampling is performed as shown in Table 7, a precoding matrix isconcentrated on a specific direction on a codebook space, therebycausing performance degradation. To overcome the issue, Table 8 belowmay be applied.

TABLE 8 PMI for W1 PMI for W2 total RI #bits values #bits values #bits 13 {0, 4, 8, 12, 16, 20, 1 {0, 2} 4 24, 28} 2 3 {0, 4, 8, 12, 16, 20, 1{0, 1} 4 24, 28} 3 0 None (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4identity matrix) 9, 10, 11, 12, 13, 14, 15} 4 0 none (W1 is 4 {0, 1, 2,3, 4, 5, 6, 7, 8, 4 identity matrix) 9, 10, 11, 12, 13, 14, 15}

As another method, in type 2 c report, a subsampling method of W1/W2 maybe applied as shown in Table 9 below. In Tables 7 and 8, W1 and W2 arerepresented in 3 bits and 1 bits, respectively, but in Table 9. W1 andW2 are represented in 2 bits and 2 bits, respectively such that W2ensures a degree of freedom for selecting a selection vector as well asa co-phasor factor. That is, e3 as well as e1 may be selected as aselection vector. A vector of W1 selected as e1 and a vector of W1selected as e3 are orthogonal to each other. When frequency selectivityis high, e1 or e3 may be selected in W2 as subband information so as toaccurately feedback a channel direction if possible.

TABLE 9 PMI for W1 PMI for W2 total RI #bits values #bits values #bits 12 {0, 2, 4, 6} 2 {0, 2, 8, 10} 4 2 2 {0, 2, 4, 6} 2 {0, 1, 4, 5} 4 3 0None (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4 identity matrix) 9, 10, 11,12, 13, 14, 15} 4 0 none (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4 identitymatrix) 9, 10, 11, 12, 13, 14, 15}

In Table 9, W1 is subsampled using the same method as in Table 6. W2 issubsampled as shown in Table 9 so as to select e1 and e3 as a selectionvector of W2. In this case, even if W1 is configured with {0,2,4,6},only e1 and e3 instead of e1, e2, e3, and e4 may be selected as theselection vector of W2. Accordingly, a final precoding matrix generatedby multiplying W1 and W2 uses only a vector with non-uniformlydistributed among 32 vectors of a 2×32 DFT matrix oversampled 16 times.That is, only {0, 2, 4, 6, 16, 18, 20, 22}th DFT vectors are used.

When subsampling is performed as shown in Table 9, a precoding matrixmay be concentrated on a specific direction on a codebook space todegrade performance. To overcome this issue, subsampling may beperformed as shown in Table 10 below. In Table 19 below, a finalprecoding matrix generated by multiplying W1 and W2 may use{0,4,8,12,16,20,24,28}th vectors with uniformly distributed values among32 vectors of a 2×32 DFT matrix oversampled 16 times.

TABLE 10 PMI for W1 PMI for W2 total RI #bits values #bits values #bits1 2 {0, 4, 8, 12} 2 {0, 2, 8, 10} 4 2 2 {0, 4, 8, 12} 2 {0, 1, 4, 5} 4 30 None (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4 identity matrix) 9, 10,11, 12, 13, 14, 15} 4 0 none (W1 is 4 {0, 1, 2, 3, 4, 5, 6, 7, 8, 4identity matrix) 9, 10, 11, 12, 13, 14, 15}

In Table 10 above, when rank is 1 and 2, {0, 4, 8, 12} as a codebookindex of W1 may be deduced by multiplying a first PMI index IPMI1 withone of 0 to 3 by four.

In addition, in Table 10, when rank is 1, {0, 2, 8, 10} as a codebookindex of W2 may be deduced by applying a second PMI index IPMI2 with oneof 0 to 3 to the following equation.

2I _(PMI2)+4·└I _(PMI2)/2┘  [Equation 15]

In addition, in Table 10, when rank is 2, {0, 1, 4, 5} as a codebookindex of W2 may be deduced by applying a second PMI index IPMI2 with oneof 0 to 3 to the following equation.

I _(PMI2)+2·└I _(PMI2)/2┘  [Equation 16]

Third Embodiment

A third embodiment of the present invention relates to another exampleof the aforementioned 4Tx codebook of Equations 12 to 15, and even ifthe codebook of the third embodiment of the present invention is used,the first and second embodiments of the present invention may beapplied. The aforementioned codebook of Equations 12 to 15 and thecodebook according to the third embodiment of the present invention arethe same except for some codewords (9, 10, 11, 12, 13, 14, 15) of W2 inrank 2. Accordingly, when the codebook according to the third embodimentof the present invention is subsampled according to the first or secondembodiment of the present invention, subsampled codebooks are the same.

4 Tx codebook according to the third embodiment of the present inventionmay be represented by multiplication of two matrices as follows.

W=W ₁ ·W ₂  [Equation 17]

Here, the inner precoder W₁ and the outer precoder W₂ may representwideband/long-term channel properties and subband/short-term channelproperties, respectively. W₁ may be set as follows.

$\begin{matrix}{{W_{1} = \begin{bmatrix}X_{n} & 0 \\0 & X_{n}\end{bmatrix}},{n = 0},1,\Lambda,15} & \left\lbrack {{Equation}\mspace{14mu} 18} \right\rbrack\end{matrix}$

Here, X_(n) may be set as follows.

$\begin{matrix}{X_{n} = {{\begin{bmatrix}1 & 1 & 1 & 1 \\q_{1}^{n} & q_{1}^{n + 8} & q_{1}^{n + 16} & q_{1}^{n + 24}\end{bmatrix}\mspace{14mu} {where}\mspace{14mu} q_{1}} = e^{j\; 2\; {\pi/32}}}} & \left\lbrack {{Equation}\mspace{14mu} 53} \right\rbrack\end{matrix}$

Codebook W₂ for rank 1 may be set as follows.

$\begin{matrix}{{W_{2,n} \in \left\{ {{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{\alpha (i)}Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{j\; {\alpha (i)}Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{- {\alpha (i)}}Y}\end{bmatrix}},{\frac{1}{\sqrt{2}}\begin{bmatrix}Y \\{{- j}\; {\alpha (i)}Y}\end{bmatrix}}} \right\}},\mspace{79mu} {Y = {{e_{i} \in {\left\{ {e_{1},e_{2},e_{3},e_{4}} \right\} \mspace{14mu} {and}\mspace{14mu} {\alpha (i)}}} = q_{1}^{2{({i - 1})}}}}} & \left\lbrack {{Equation}\mspace{14mu} 19} \right\rbrack\end{matrix}$

Codebook W2 for rank 2 may be set as follows.

$\begin{matrix}{{W_{2,n} \in {\left\{ {{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & Y_{2}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\{- Y_{1}} & Y_{2}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\{- Y_{1}} & {- Y_{2}}\end{bmatrix}}} \right\} \left( {Y_{1},Y_{2}} \right)} \in \left\{ \left( {e_{2},e_{4}} \right) \right\}}\mspace{85mu} {and}{W_{2,n} \in {\left\{ {{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{1} & {- Y_{2}}\end{bmatrix}},{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\{j\; Y_{1}} & {{- j}\; Y_{2}}\end{bmatrix}}} \right\} \left( {Y_{1},Y_{2}} \right)} \in \left\{ {\left( {e_{1},e_{1}} \right),\left( {e_{2},e_{2}} \right),\left( {e_{3},e_{3}} \right),\left( {e_{4},e_{4}} \right)} \right\}}\mspace{79mu} {and}{W_{2,n} \in {\left\{ {{\frac{1}{2}\begin{bmatrix}Y_{1} & Y_{2} \\Y_{2} & {- Y_{1}}\end{bmatrix}},} \right\} \left( {Y_{1},Y_{2}} \right)} \in \left\{ {\left( {e_{1},e_{3}} \right),\left( {e_{2},e_{4}} \right),\left( {e_{3},e_{1}} \right),\left( {e_{4},e_{2}} \right)} \right\}}} & \left\lbrack {{Equation}\mspace{14mu} 20} \right\rbrack\end{matrix}$

Here, e_(n) is a 4-element selection vector with all zeros except forthe nth element with 1.

An index of W1 is defined as i1, and i1 is the same as index n of W1 inthe aforementioned equation of 4Tx codebook.

In addition, an index of W2 is defined as shown in the following table.

TABLE 4 Index of W2 W2 for rank 1 W2 for rank 2  0$\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{q_{1}^{0}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{1} \\e_{1} & {- e_{1}}\end{bmatrix}$  1 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{jq}_{1}^{0}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{1} \\{je}_{1} & {- {je}_{1}}\end{bmatrix}$  2 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{- q_{1}^{0}}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{2} \\e_{2} & {- e_{2}}\end{bmatrix}$  3 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{1} \\{{- {jq}_{1}^{0}}e_{1}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{2} \\{je}_{2} & {- {je}_{2}}\end{bmatrix}$  4 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{q_{1}^{2}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{3} & e_{3} \\e_{3} & {- e_{3}}\end{bmatrix}$  5 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{jq}_{1}^{2}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{3} & e_{3} \\{je}_{3} & {- {je}_{3}}\end{bmatrix}$  6 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{- q_{1}^{2}}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{4} & e_{4} \\e_{4} & {- e_{4}}\end{bmatrix}$  7 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{2} \\{{- {jq}_{1}^{2}}e_{2}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{4} & e_{4} \\{je}_{4} & {- {je}_{4}}\end{bmatrix}$  8 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{q_{1}^{4}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\e_{2} & e_{4}\end{bmatrix}$  9 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{jq}_{1}^{4}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\e_{2} & {- e_{4}}\end{bmatrix}$ 10 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{- q_{1}^{4}}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\{- e_{2}} & e_{4}\end{bmatrix}$ 11 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{3} \\{{- {jq}_{1}^{4}}e_{3}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\{- e_{2}} & {- e_{4}}\end{bmatrix}$ 12 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{q_{1}^{6}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{1} & e_{3} \\e_{3} & {- e_{1}}\end{bmatrix}$ 13 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{jq}_{1}^{6}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{2} & e_{4} \\e_{4} & {- e_{2}}\end{bmatrix}$ 14 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{- q_{1}^{6}}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{3} & e_{1} \\e_{1} & {- e_{3}}\end{bmatrix}$ 15 $\frac{1}{\sqrt{2}}\begin{bmatrix}e_{4} \\{{- {jq}_{1}^{6}}e_{4}}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}e_{4} & e_{2} \\e_{2} & {- e_{4}}\end{bmatrix}$

With reference to FIG. 16, a channel state information reporting methodwill be described according to an embodiment of the present invention.With reference to FIG. 16, a channel state information reporting methodaccording to an embodiment of the present invention will be described.

In operation S161, a UE subsamples a first codebook associated with afirst precoding matrix indicator (PMI) and a second codebook associatedwith a second PMI according to a report submode for a 4 antenna port.

A detailed subsampling method is the same as the subsampling methoddescribed with regard to the second embodiment of the present inventionand a detailed description thereof will be omitted.

In operation S163, the UE reports channel state information based on thesusampled first codebook and second codebook.

Here, when a rank indicator (RI) is 1 or 2, a first codebook index forthe first PMI is determined as one of 0, 4, 8, and 12. When the RI is 1,a second codebook index for the second PMI is determined as one of 0, 2,8, and 10. When the RI is 2, the second codebook index for the secondPMI is determined as one of 0, 1, 4, and 5.

With regard to the channel state information transmitting method, theaforementioned various embodiments of the present invention areindependently applied or two or more embodiments are simultaneouslyapplied and descriptions of redundant parts are omitted for clarity.

In addition, the same idea as that proposed by the present invention canalso be applied to uplink MIMO transmission and reception for MIMOtransmission between a BS and a relay (in backhaul uplink and backhauldownlink) and MIMO transmission between a relay and a UE (in accessuplink and access downlink).

BS and UE to which embodiments of the present invention are applicable

FIG. 17 is a diagram illustrating a BS 110 and a UE 120 to which anembodiment of the present invention is applicable.

When a relay is included in a wireless communication system,communication in backhaul link is performed between the BS and therelay, and communication in access link is performed between the relayand the UE. Accordingly, the BS or the UE illustrated in the drawing maybe replaced by a relay as necessary.

Referring to FIG. 17, the wireless communication system includes a BS1710 and a UE 1720. The BS 1710 includes a processor 1712, a memory1714, and a radio frequency (RF) unit 1716. The processor 1712 may beconfigured to embody procedures and/or methods proposed by the presentinvention. The memory 1714 is connected to the processor 1712 and storesvarious information related to an operation of the processor 1712. TheRF unit 1716 is connected to the processor 1712 and transmits and/orreceives a radio signal. The UE 1720 includes a processor 1722, a memory1724, and an RF unit 1726. The processor 1722 may be configured toembody procedures and/or methods proposed by the present invention. Thememory 1724 is connected to the processor 1722 and stores variousinformation related to an operation of the processor 1722. The RF unit1726 is connected to the processor 1722 and transmits and/or receives aradio signal. The BS 1710 and/or the UE 1720 may have a single antennaor a multiple antenna. The embodiments of the present inventiondescribed hereinbelow are combinations of elements and features of thepresent invention. The elements or features may be considered selectiveunless otherwise mentioned. Each element or feature may be practicedwithout being combined with other elements or features. Further, anembodiment of the present invention may be constructed by combiningparts of the elements and/or features. Operation orders described inembodiments of the present invention may be rearranged. Someconstructions of any one embodiment may be included in anotherembodiment and may be replaced with corresponding constructions ofanother embodiment. It is obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by a subsequent amendment after theapplication is filed.

In the embodiments of the present invention, a specific operationdescribed as being performed by the BS may be performed by an upper nodeof the BS. Namely, it is apparent that, in a network comprised of aplurality of network nodes including a BS, various operations performedfor communication with a UE may be performed by the BS, or network nodesother than the BS. The term ‘BS’ may be replaced with a fixed station, aNode B, an eNode B (eNB), an access point, etc.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or combinationthereof. In a hardware configuration, the embodiments of the presentinvention may be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

In a firmware or software configuration, the embodiments of the presentinvention can be implemented by a type of a module, a procedure, or afunction, which performs functions or operations described above.Software code may be stored in a memory unit and then may be executed bya processor.

The memory unit may be located inside or outside the processor totransmit and receive data to and from the processor through variousmeans which are well known.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

The above-described embodiments of the present invention can be appliedto a wireless communication system such as a user equipment (UE), arelay, a base station (BS), etc.

What is claimed is:
 1. A method for receiving channel state information (CSI) by a base station in a wireless communication system, the method comprising: transmitting configuration information indicating whether to use a 4 antenna ports codebook, wherein a first codebook index for a first precoding matrix indicator (PMI) and a second codebook index for a second PMI for a specific CSI report mode with 4 antenna ports are determined when the UE is configured to use the 4 antenna ports codebook; and receiving the CSI indicating the first codebook index and the second codebook index, wherein a value of the first codebook index is determined as follows: when a rank indicator (RI) is 1, the value of the first codebook index is determined based on a value of the first PMI multiplied by 4, when the RI is 2, the value of the first codebook index is determined based on the value of the first PMI multiplied by 4, and when the RI is 3 or 4, a precoding matrix corresponding to the first PMI is an identity matrix.
 2. The method of claim 1, wherein a value of the second codebook index is determined as follows: when the RI is 1, the value of the second codebook index is determined as one of 0, 2, 8, and 10, when the RI is 2, the value of the second codebook index is determined as one of 0, 1, 4, and 5, and when the RI is 3 or 4, the value of the second codebook index is determined as one of integers from 0 to
 15. 3. The method of claim 1, wherein, when the RI is 1 or 2, each of the value of the first PMI and the value of the second PMI is represented by 2 bits.
 4. The method of claim 1, wherein a total payload size for indicating the first codebook index and the second codebook index is 4 bits.
 5. The method of claim 1, wherein the specific CSI report mode is a specific submode of a periodic CSI report mode.
 6. The method of claim 1, wherein when the RI is 1 or 2, a final precoding matrix is to be determined based on the first codebook index and the second codebook index.
 7. The method of claim 1, wherein the CSI further includes a wideband channel quality indicator (CQI).
 8. The method of claim 7, wherein the first codebook index, the second codebook index and the wideband CQI are reported in a same subframe.
 9. The method of claim 8, wherein the RI is reported in a subframe different from the same subframe.
 10. A base station for receiving channel state information (CSI) in a wireless communication system, the base station comprising: a transceiver; and a processor operably coupled with the transceiver and configured to: transmit configuration information indicating whether to use a 4 antenna ports codebook, wherein a first codebook index for a first precoding matrix indicator (PMI) and a second codebook index for a second PMI for a specific CSI report mode with 4 antenna ports are determined when the UE is configured to use the 4 antenna ports codebook; and receive the CSI indicating the first codebook index and the second codebook index, wherein a value of the first codebook index is determined as follows: when a rank indicator (RI) is 1, the value of the first codebook index is determined based on a value of the first PMI multiplied by 4, when the RI is 2, the value of the first codebook index has the value same as the value of the first PMI multiplied by 4, and when the RI is 3 or 4, a precoding matrix corresponding to the first PMI is an identity matrix.
 11. The base station of claim 10, wherein a value of the second codebook index is determined as follows: when the RI is 1, the value of the second codebook index is determined as one of 0, 2, 8, and 10, when the RI is 2, the value of the second codebook index is determined as one of 0, 1, 4, and 5, and when the RI is 3 or 4, the value of the second codebook index is determined as one of integers from 0 to
 15. 12. The base station of claim 10, wherein, when the RI is 1 or 2, each of the value of the first PMI and the value of the second PMI is represented by 2 bits.
 13. The base station of claim 10, wherein a total payload size for indicating the first codebook index and the second codebook index is 4 bits.
 14. The base station of claim 10, wherein the specific CSI report mode is a specific submode of a periodic CSI report mode.
 15. The base station of claim 10, wherein when the RI is 1 or 2, a final precoding matrix is to be determined based on the first codebook index and the second codebook index.
 16. The base station of claim 10, wherein the CSI further includes a wideband channel quality indicator (CQI).
 17. The base station of claim 16, wherein the first codebook index, the second codebook index and the wideband CQI are reported in a same subframe.
 18. The base station of claim 17, wherein the RI is reported in a subframe different from the same subframe. 